Firefox manifest warnings from Chrome-only permissions

[ryan-carpenter]

Firefox reports three warnings of type "Reading manifest: Warning processing permissions:"

Error processing permissions.0: Value "<all_urls>"

<all_urls> is already in host_permissions and does not belong in permissions

Error processing permissions.7: Value "offscreen"

offscreen is only implemented in Chrome-based browsers.

Warning processing key: An unexpected property was found in the WebExtension manifest.

The unexpected property is key (the extension ID for Chrome-based browser).
The extension ID for Firefox (@jabfox) is in browser_specific_settings.gecko.id

Firefox 152.0.6 (aarch64) macOS.

I used Claude to troubleshoot. A pull request is on the way.